When the workpiece on such a machine is rotated at a constant angular speed in order for the tool e.g. grinding wheel to form the required profile, the stock removal rate and surface finish may vary according to the profile cut and tool or grinding wheel geometry.
In the particular, though not exclusive case, of a camshaft grinding machine, the problem is particularly acute as detailed below.
FIG. 6 of the accompanying drawings shows a cam profile 127 being ground by grinding wheel 18. Normally the cam is rotated at a constant angular speed .theta.. This results in large variation of linear speed V of the profile past the wheel. The geometry of the wheel and cam effect the following ramp 121 is 17% of the length of the cam profile, yet is ground during only 5.degree. of cam rotation, or 1.4% of the total angular rotation. If base circle 120 is taken as a linear speed of unity then ramps 121 and 125 have linear speeds up to thirty times the base circle while corners 122 and 124 may be one third of the base circle speed. As stock removal rate is directly proportional to the linear speed of the profile past the wheel then if the cam is rotated at constant angular speed the stock removal rate will vary considerably.
In order to achieve constant stock removal rate, therefore, the angular speed of the cam should be varied as the cam rotates such that the linear speed V is constant. For a typical cam profile as shown, .theta. will be constant while the base circle 120 is ground, the ramps 121 and 125 require .theta. to be reduced by up to thirty times, peak 123 requires a reduction of up to one and a half times, and corners 122 and 124 require .theta. to be up to three times greater than base circle .theta..